The present invention relates generally to ignition systems in spark ignited engines, and more particularly relates to such systems in lean burn engines in which the excess air factor xcex is greater than 1.4.
Industry has developed various techniques using ionization signals for detecting abnormal combustion conditions such as misfire, knock, and approximate air/fuel ratio for stochiometric engines . Free ions present in the combustion gases are electrically conductive and are measurable by applying a voltage across an ionization probe. Alternatively, the voltage is applied across the electrodes of a spark plug after the spark plug has ignited the combustion mixture. The applied voltage induces a current in the ionized gases which is measured to provide the ionization signal. The ionization signal is used as a control parameter in the control of the engine. For example, in U.S. Pat. No. 6,029,627, ionization signals and a single O2 sensor in the exhaust are used to control the air/fuel ratio in engines to achieve stoichiometric operation. This technique uses the O2 sensor to achieve stoichiometry of the overall stoichiometric mixture of the engine and then equalizes the amplitude or location of the first local peak of the ionization signal in each individual cylinder. Another technique disclosed in U.S. Pat. No. 5,992,386 performs a frequency analysis of the ionization signal to detect abnormal combustion conditions such as knock. These systems integrate the ionization signal and compare the magnitude of the integrated signal to the magnitude of the integrated signal of a normal combustion event. The abnormal combustion condition is detected if the magnitude of the integrated signal is above a threshold level, which is set above the magnitude of the integrated signal of a normal combustion event.
One of the drawbacks of stochiometric engines is the emission of pollutants. With fixed engine timing and load, the NOx, emissions level of a typical gas engine is dependent upon the air/fuel ratio. Near a chemically correct (i.e., stoichiometric) ratio, the NOx, emissions peak and then drop significantly as the amount of excess air is increased. Maintaining a stable combustion process with a high air/fuel ratio is difficult to manage. As a result, conventional spark-ignited gas engines typically operate near the stoichiometric air/fuel ratio and depend upon exhaust after treatment with catalytic converters to reduce the NOx emissions.
Government agencies and industry standard setting groups are reducing the amount of allowed emissions in an effort to reduce pollutants. As a result, industry is moving towards using lean burning engines to reduce emissions despite the difficulty of maintaining a stable combustion process in lean burning engines. By using more air during combustion, turbocharged lean-bum engines can enhance fuel efficiency without sacrificing power and produce less nitrous oxide pollutants than conventional stoichiometric engines.
Ionization sensing has not been utilized to any significant extent in these lean burn engines. Because of the lean nature of the mixture, the ionized species concentration, including NOx, is much less than at stoichiometric conditions. As a result, the ionization signal is of very low intensity and has great variability. The techniques developed using ionization signals for stochiometric operation are unsuitable for lean burn operation and do not work. For example, the ionization signals of some lean burn engines are sufficiently variable and are low enough in magnitude that integrating the signal can not be done reliably due to a number of factors. These factors include higher levels of noise relative to the ionization signal magnitude, the variability of the ionization signal, and the low magnitudes of the resultant integrated signal.
In view of the foregoing, an object of the present invention is to reliably detect abnormal combustion conditions such as misfire and knock of lean burn engines based on ionization signals.
The foregoing objects are among those attained by the invention, which provides a method of detecting an abnormal combustion condition in a combustion chamber of a lean burn reciprocating engine. The abnormal combustion condition includes misfire and knock. The method measures the variation of an ionization signal that changes with respect to an engine parameter over a combustion event of the lean burn reciprocating engine operating with an air to fuel ratio corresponding to a lambda (xcex) greater than 1.4, associates a floating bounded space with the ionization signal, determines if a portion of the ionization signal is within the floating bounded space, and provides an indication that the abnormal combustion condition has been detected if the portion of the ionization signal is within the floating bounded space.
A method to determine the floating bounded space and a starting point for the floating bounded space is also disclosed. The method includes receiving a set of ionization signals that change with respect to an engine parameter over a combustion event. The set of ionization signals has ionization signals corresponding to normal combustion conditions and ionization signals corresponding to at least one abnormal combustion condition for an engine operating with an air to fuel ratio corresponding to a xcex greater than 1.4. The method further includes the step of adjusting the starting point and size of the floating bounded space such that selected portions of the ionization signals corresponding to the at least one abnormal combustion condition reliably fall within the floating bounded space and the ionization signals corresponding to normal combustion conditions reliably fall outside the floating bounded space.